This invention relates to a method for making a plastic container, and more particularly to a method of manufacturing a cup-shaped container with a flange which is improved in transparency, strength, gas barrier properties and wall thickness distribution, and has a small wall thickness and a large ratio of height to diameter.
A plug-assist air-pressure forming method has been proposed and practically employed in the art in order to form a cup-shaped container which is excellent in transparency, strength and gas barrier properties and small in wall thickness. In the method, while the peripheral portion of a blank which is made of a thermoplastic resin sheet is restrained, a plug is pushed into a die cavity and at the same time compressed air is supplied or vacuum is applied; that is, drawing and expanding processes are employed in combination, to form a cup-shaped container. Since an expanding process is included in the method, if the forming depth is great, the thickness of the side wall portion, especially the corner portion at the lower end of the side wall portion is reduced, and accordingly these portion are liable to break readily. Thus, it is difficult to manufacture a plastic container having a side wall portion or a bottom wall portion which is uniform in thickness. In other words, it is substantially impossible to form a cup-shaped container whose height is larger than its diameter, according to this prior art method.
Furthermore, a method of manufacturing a cup-shaped container with a flange has been disclosed in the specification of U.S. Pat. No. 4,122,147. In this method, thermoplastic plastic resin at an elevated temperature is compressed between a lower plunger having an outside diameter substantially equal to the inside diameter of the die cavity and an upper plunger whose diameter is smaller by the amount of desired thickness of the container side wall portion. This compression squeezes the plastic resin into the peripheral groove at the upper end portion of the cavity thereby to form the flange portion. While the plastic resin is being squeezed from between the upper and lower plungers, the upper and lower plungers are moved down the cavity to form the side wall portion, thus forming the cup-shaped plastic container with a flange portion. In this method, as indicated in the example in the specification, the formation is carried out with the plastic resin in molten state at a far higher temperature than its melting point (polyethylene powder having a density of about 0.95 g/cc (whose melting point is about 130.degree. C.) being heated to about 180.degree. C., and the upper plunger compressing the plastic resin being at 176.degree. C.) and at high fluidity. A large quantity of filler such as mica is added in order to improve the gas barrier characteristic of the formed container. Accordingly, it is difficult to manufacture a container excellent in transparency and strength. In view of the gas barrier characteristic and strength, it is difficult to provide a container small in wall thickness. The specification indicates that the thickness of the side wall portion is 0.8 mm. In the method, the container is formed under the condition that the flange portion is not pressed in the vertical direction. Therefore, in the case where the forming temperature is high as described above, burrs are liable to be created on the peripheral edge of the flange portion. On the other hand, in the case where the forming temperature is close to the melting point or lower than the melting point, the flange portion is liable to be wrinkled.